It is known that many alcohols can be catalytically dehydrogenated to the corresponding carbonyl compounds (for general information see Hydlicky Milos, Oxidations in Organic Chemistry, ACS Monograph 186, American Chemical Society, Washington, D.C., 1990, p. 132 and Smith M. B. and March J. Advanced Organic Chemistry, 5th edition, John Wiley and Sons, Inc., New York, 2001, pp. 1515-1516). Usually, dehydrogenation of alcohols allows preparation of the target carbonyl compounds with high yields and throughputs. To that end, copper, nickel, and palladium based catalysts have been typically used to carry out the dehydrogenation of alcohols.
An attempt to produce alpha, beta-unsaturated cyclic ketones, such as carvone, by catalytic dehydrogenation of a corresponding secondary allylic cyclic alcohol, such as carveol, was made in 1927 (Treibs W. and Schmidt H, Ber., 1927, 60 B, pp. 2335-2341). However, this attempt failed as both copper and nickel based catalysts converted the carveol into carvacrol and tetrahydrocarvone, but not to carvone. This was due, in part, to two potential side reactions that can accompany the catalytic dehydrogenation of a secondary allylic cyclic alcohol, such as carveol. First, both the starting material and the product possess double bonds, which can react with hydrogen that is produced as a result of dehydrogenation. Secondly, these double bonds easily isomerize at high temperature in the presence of catalysts to provide an aromatic structure. The scheme below represents these side reactions that can occur during, for example, the dehydrogenation of carveol. 
Prior to the discovery of the present invention, most known methods for the preparation of alpha, beta-unsaturated cyclic ketones from a corresponding secondary allylic cyclic alcohol, such as the preparation of carvone from carveol, involve some type of an oxidation reaction. These methods can be divided in two categories.
The first of these two methods is known as the Oppenauer oxidation, where hydrogen is transferred from carveol to an auxiliary carbonyl compound. Japanese patent JP 50/58031 describes carveol oxidation in the presence of aluminum isopropoxide as catalyst, cyclohexanone as hydrogen acceptor, and xylene as solvent. The yield of the 88% pure carvone was 82%. A better yield of carvone (91%) was obtained by employing of a complex aluminum catalyst and three equivalents of pivalaldehyde as hydrogen acceptor in a methylene chloride solution (Takashi Ooi, et al, Synthesis, 2002, No. 2, pp. 279-291). The new aluminum complex catalyst used in this method (2,7-dimethyl-1,8-biphenyldioxy)bis(dialkoxyaluminum) has to be prepared from trialkylaluminum, which imposes safety concerns on an industrial scale. Common disadvantages of all Oppenauer type oxidation methods include the catalyst sensitivity toward hydrolysis, the necessity of use of an auxiliary carbonyl compound (sometimes a large excess) and a lengthy and labor intensive work-up.
The second of these methods is known as oxidation with a reagent. Above mentioned Japanese patent JP 50/58031 also describes carveol oxidation to carvone with chromic trioxide in concentrated sulfuric acid with 93% yield. Among other reagents suggested for carveol oxidation to carvone are, hydrogen peroxide in the presence of molybdenum catalyst (Trost, B. M. et al., Israel Journal of Chemistry, 1984, Vol. 24, pp. 134-143); N-methylmorpholine-N-oxide in the presence of ruthenium catalyst (Sharpless K. B. et al., Tetrahedron Letters, 1976, No. 29, pp. 2503-2506); hydroperoxides in the presence of molybdenum and vanadium catalysts (Lempers H. E. B. et al., J. Org. Chem., 1998, Vol. 63, pp. 1408-1413); and copper catalysts (Rothenberg G., J. Chem. Soc., Perkin Trans. 1998, No. 2, pp. 2429-2434). In most, if not all, of these reactions expensive reagents or toxic catalysts are used and a large excess of the oxidation reagent is required, which makes a reagent oxidation very unattractive for commercialization.
The double bond in carvone that is conjugated with the carbonyl group is markedly active as hydrogen acceptor. This is why under commonly used dehydrogenation conditions dihydrocarvone becomes the major product of carveol dehydrogenation (see, for example, U.S. Pat. No. 4,160,786 which describes isomerization of cycloalkenols to cycloalkanones in the presence of copper-chromite catalysts and specifically mentions carveol conversion to dihydrocarvone). Supported palladium, platinum and ruthenium catalysts, which are frequently used in dehydrogenation reaction, afford phenols and cyclohexanones upon dehydrogenation of cycloalkenols (carveol) or cycloakenones (carvone). Examples of such transformations can be found in U.S. Pat. No. 4,929,762 and U.S. Pat. No. 5,817,891.
In some instances a method called oxidative dehydrogenation is employed to produce alpha,beta-unsaturated carbonyl compounds from the corresponding allylic alcohols. Catalysts utilized in this process include metallic copper or silver. Using this process geraniol was converted to citral (U.S. Pat. No. 5,241,122) and prenol to prenal (U.S. Pat. No. 6,013,843) at the temperature above 360° C. The name of this process—oxidative dehydrogenation—suggests that this is not a true dehydrogenation, as it requires the presence of oxygen, which could be either an oxidant or a hydrogen acceptor. Nonetheless, the oxidative dehydrogenation has never been successfully used to produce carvone, probably because it proceeds at the temperature above 360° C., which causes decomposition of carveol and carvone and leads to low yields and poor quality.
In other attempts, some enzymes have been found to affect this kind of chemical transformation (Hirata, T., et al., Phytochemistry, 2000, vol. 55, No. 4, pp. 297-303). The enzymatic method has mostly a theoretical interest and cannot be used for a large-scale production of carvone.
In general, homogeneous catalysts are rarely used in dehydrogenation process (Blum, J., Biger, S. Tetrahedron Letters, 1970, No. 21, pp. 1825-1828). In particular, in the presence of those homogeneous catalysts that could possibly affect dehydrogenation of the allylic alcohols the isomerization to saturated carbonyl compounds but not dehydrogenation to corresponding unsaturated carbonyl compounds was observed (see review by van der Drift, R. C. et al., J. Organomet. Chem., 2000, No. 650, pp. 1-24). There are a few examples of the homogeneous dehydrogenation of alcohols. However, only saturated alcohols were used as substrates (Fragale, C. et al. J. Molecular Catalysis, 1979, Vol. 5, pp. 65-73). Interestingly, most of the reported examples were not dehydrogenation, but rather hydrogen transfer reactions, which involved hydrogen acceptors. Thus, there is no indication in the patent or scientific literature that carvone or any other conjugated alpha, beta-unsaturated cyclic ketones can be prepared by catalytic dehydrogenation of the corresponding allylic alcohol.
Moreover, the homogeneous dehydrogenation catalysts that have been used are complex compounds of the transition metals chosen from groups six to ten of the Periodic Table. In contrast, the present invention further provides a method that utilizes carboxylates of the metals chosen from groups two and twelve of the Periodic Table. As discussed below, and in accordance with the present invention, these carboxylates are effective homogeneous dehydrogenation catalysts that allow for selective production of alpha, beta-unsaturated cyclic ketones from the corresponding secondary allylic cyclic alcohol via a true dehydrogenation mechanism.